Coal-series gas carbon dioxide production increasing simulation method based on low-field nuclear magnetic resonance technology

Abstract

The invention discloses a coal-series gas carbon dioxide production increasing simulation method based on a low-field nuclear magnetic resonance technology, and belongs to the technical field of coal-series gas development. The method specifically comprises the following steps: (1) simulating gas bearing conditions of different lithologic reservoirs under a waste condition; (2) identifying the occurrence state of polymorphic CH4 in each lithologic reservoir based on a low-field nuclear magnetic resonance technology; (3) CO2 is injected into all the lithologic reservoirs, and the CO2 air inflow in all the lithologic reservoirs is recorded in real time; (4) quantitatively representing the phase change of the multiphase fluid along with the CO2 injection time after the CO2 enters each lithologic reservoir based on the dynamic quantification of the multiphase fluid; and (5) constructing a response surface model, and determining the optimal injection condition of CO2. According to the method, the CO2 injection amount, the multi-state CH4 occurrence state change and the multi-phase fluid phase evolution in different lithologic reservoirs under the waste condition are monitored in real time, data support is provided for optimizing parameters such as CO2 injection pressure, temperature and duration, and the double targets of efficient utilization of coal series gas residual resources and CO2 geological sequestration are achieved.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane development technology, and more specifically to a simulation method for enhanced carbon dioxide recovery of coalbed methane based on low-field nuclear magnetic resonance technology. Background Technology

[0002] Coal-series gas is a natural gas resource with methane as its main component. It includes various types such as coalbed methane, coal rock gas, and tight sandstone gas, and is formed during the coal-forming process of coal-series strata.

[0003] Currently, the development of coal-bearing gas faces two major core contradictions: Reservoir complexity: Coal-bearing strata are usually composed of multiple lithologies such as coal, shale, and tight sandstone. The pore structure, permeability and gas occurrence state of each lithology reservoir are significantly different, making it difficult to unify development plans. Efficiency degradation in the later stages of development: In abandoned wells or reservoirs in the later stages of development, residual gas (adsorbed gas, gas encapsulated by bound water) accounts for more than 60%, which is difficult to effectively utilize using conventional depressurization desorption methods.

[0004] Traditional technologies (such as depressurization desorption and hydraulic fracturing) suffer from high water consumption, high risk of secondary pollution, and low displacement efficiency. Among these, competitive carbon dioxide adsorption-enhanced extraction (CO2-ECBM), which can replace adsorbed methane (CH4) with CO2 and achieve geological storage, is considered a key means to solve the aforementioned problems. However, this technology has the following drawbacks: Lithological differences were ignored: CO2-CH4 in reservoirs of different lithologies such as coal and shale were not distinguished. Differences in competitive adsorption mechanisms lead to biases in the evaluation of enhanced oil recovery effects; Dynamic monitoring is lacking: there is a lack of monitoring of multiphase fluids (free gas, adsorbed gas, dissolved gas, hydrates) along with CO2. Real-time quantification of the injection process makes it difficult to optimize injection parameters; Phase change ambiguity: CO2 After injection, the evolution of the gas-water-solid three-phase interface within the reservoir is unclear, which can easily lead to secondary damage such as water lock and bituminous deposition.

[0005] Low-field nuclear magnetic resonance (LF-NMR) is a nuclear magnetic resonance technique that operates in magnetic field strengths from 0.1T to 1.5T. It has advantages such as low cost, portability, and specific applications. With its non-destructive, dynamic, and high-resolution monitoring capabilities, it has become a key tool for revealing the competitive adsorption process of CO2-CH4 and optimizing the enhanced oil recovery process.

[0006] Therefore, how to develop a simulation method for enhanced carbon dioxide recovery of coal-bearing gas based on low-field nuclear magnetic resonance technology is an urgent problem that needs to be solved by those skilled in the art. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a simulation method for enhanced carbon dioxide recovery of coal-bearing gas based on low-field nuclear magnetic resonance technology, so as to overcome the shortcomings of the prior art.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A simulation method for enhanced carbon dioxide recovery of coal-bearing gas based on low-field nuclear magnetic resonance technology specifically includes the following steps: (1) The gas content of different lithological reservoirs under simulated abandoned conditions was determined by low-field nuclear magnetic resonance T2 spectrum test combined with centrifugation method. (2) Based on low-field nuclear magnetic resonance technology, a low-field nuclear magnetic resonance analyzer was used to perform CPMG sequence scanning on each lithological reservoir to identify the occurrence state of polymorphic CH4 in each lithological reservoir; (3) Inject CO2 into each lithological reservoir, record the CO2 intake in each lithological reservoir in real time through flow sensors, collect T2 spectrum at the same time, and monitor the change in the intensity of short T2 peak signal representing CO2. (4) Based on the dynamic quantification of multiphase fluids, quantitatively characterize the phase changes of multiphase fluids with CO2 injection time after CO2 enters each reservoir stratum; (5) Construct a response surface model to determine the optimal CO2 injection conditions.

[0009] Furthermore, in step (1) above, the lithological reservoir includes coal, shale and tight sandstone.

[0010] Furthermore, in step (1) above, the centrifugation speed is 10,000 rpm and the centrifugation time is 2 hours.

[0011] Furthermore, in step (2) above, the magnetic field strength of the low-field nuclear magnetic resonance analyzer is 0.3T and the resonance frequency is 12MHz.

[0012] Furthermore, in step (2) above, polymorphic CH4 includes adsorbed CH4 and free CH4.

[0013] Furthermore, in step (3) above, the CO2 injection scheme includes pressure gradient experiments and temperature gradient experiments.

[0014] Furthermore, the operation of the pressure gradient experiment is as follows: the temperature is fixed at 50℃, and CO2 is injected sequentially at pressures of 2MPa, 4MPa, 6MPa and 8MPa, with each pressure point maintained for 2 hours.

[0015] Furthermore, the specific operation of the above temperature gradient experiment is as follows: the injection pressure is fixed at 6MPa, the temperature is adjusted to 40℃, 60℃, and 80℃, and each temperature point is maintained for 2 hours.

[0016] Furthermore, in step (4) above, the multiphase fluid includes adsorbed CO2, adsorbed CH4, free CO2 and free CH4.

[0017] Furthermore, in step (5) above, the factors in the response surface model include injection pressure, temperature, and time.

[0018] As can be seen from the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows: 1. Enhanced lithological targeting: For the first time, the CO2-CH4 competitive adsorption mechanism of coal, shale, and sandstone is distinguished, with residual gas utilization rate assessment error <8%; 2. Improved dynamic monitoring accuracy: The volume proportion of multiphase fluids is quantified by combining T2 spectroscopy and MRI, with a phase identification accuracy of 92%. 3. Optimized development efficiency: Reduces the number of trials and errors by more than 40%, and increases single-well production efficiency by 18%-25%; 4. Significant environmental benefits: The quantitative assessment error of CO2 storage efficiency is <10%, and the storage efficiency reaches 0.35t CO2 / t CH4.

[0019] 5. This invention constructs a physical model of multi-lithological reservoirs, combines low-field nuclear magnetic resonance T2 spectral analysis and MRI imaging technology, and monitors in real time the changes in CO2 injection amount, multi-state CH4 occurrence state and multi-phase fluid phase evolution in reservoirs of different lithologies under abandoned conditions. This provides data support for optimizing parameters such as CO2 injection pressure, temperature and duration, and achieves the dual objectives of efficient utilization of coalbed methane residual resources and geological CO2 sequestration.

[0020] 6. This invention employs low-field nuclear magnetic resonance technology, which can monitor H proton signals in reservoirs in real time (reflecting the state of fluid existence), providing technical support for revealing the competitive adsorption of CO2-CH4 and the evolution of multiphase fluids in multilithological reservoirs.

[0021] 7. This invention reveals the series mass transfer mechanism of "adsorption-desorption-diffusion" in the multiphase fluid within the multi-dimensional space after CO2 enters a depleted coal-bearing reservoir, thereby elucidating the CO2... Injection-induced multiphase fluid interactions and mechanisms. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating a simulation method for enhanced carbon dioxide recovery of coal-bearing gas based on low-field nuclear magnetic resonance technology. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0024] A simulation method for enhanced carbon dioxide recovery of coal-bearing gas based on low-field nuclear magnetic resonance technology, such as Figure 1 As shown, the specific steps include: (1) The gas content of different lithological reservoirs such as coal, shale and tight sandstone under simulated abandoned conditions was determined by low field nuclear magnetic resonance T2 spectrum test and centrifugation method, with centrifugation speed set at 10000 rpm and centrifugation time at 2 h. (2) Based on low-field nuclear magnetic resonance technology, a low-field nuclear magnetic resonance analyzer was used, with the magnetic field strength set to 0.3T and the resonance frequency set to 12MHz. CPMG sequence scanning was performed on each lithological reservoir to identify the occurrence state of multiple states of CH4, such as adsorbed CH4 and free CH4, in each lithological reservoir. (3) Inject CO2 into each lithological reservoir, record the CO2 intake in each lithological reservoir in real time through flow sensors, collect T2 spectrum at the same time, and monitor the change in the intensity of short T2 peak signal representing CO2. The CO2 injection scheme includes pressure gradient experiments and temperature gradient experiments. The specific procedure for the pressure gradient experiment is as follows: the temperature is fixed at 50℃, and CO2 is injected sequentially at pressures of 2MPa, 4MPa, 6MPa, and 8MPa, with each pressure point maintained for 2 hours; The specific procedure for the temperature gradient experiment is as follows: fix the injection pressure at 6MPa, adjust the temperature to 40℃, 60℃, and 80℃, and maintain each temperature point for 2 hours; (4) Based on the dynamic quantitative analysis of multiphase fluids such as adsorbed CO2, adsorbed CH4, free CO2 and free CH4, the phase changes of multiphase fluids after CO2 enters each reservoir layer with the time of CO2 injection are quantitatively characterized. (5) Construct a three-factor, three-level response surface model of injection pressure, temperature and time to determine the optimal injection conditions for CO2. Performance testing

[0025] Based on the simulation method in Example 1, simulation tests were conducted on the co-mining of coal seam No. 3 in the Qinshui Basin and shale in the Shanxi Formation, and on the interbedded coal-sandstone formation in the Xishanyao Formation of the Junggar Basin.

[0026] 1. Simulation test of co-mining of No. 3 coal seam and Shanxi Formation shale in Qinshui Basin Sample preparation: Coal and rock samples from coal seam #3 and shale from the Shanxi Formation were collected from a well in the Qinshui Basin and prepared into cylinders with a diameter of 25 mm and a height of 50 mm. The initial gas content of the coal and rock samples was 12.5 cm³. 3 / g, shale 8.2cm 3 / g; Disposal condition simulation: The sample was depressurized to 3 MPa to simulate the pressure decay in the later stages of development; CO2 injection experiment: CO2 was synchronously injected into coal and shale at 6MPa and 60℃ for 12 hours. Effect evaluation: The utilization rate of adsorbed gas in coal and rock reached 68%, and the utilization rate of free gas in shale reached 55%; the CO2 sequestration efficiency in coal and rock sections was 32%, and in shale sections it was 41%; MRI imaging showed that the water-locked area accounted for 15% in coal and rock, while there was no water-locked phenomenon in shale.

[0027] 2. Simulation test of coal-sandstone interbedded formation in Xishanyao Formation, Junggar Basin Sample preparation: Coal-sandstone interbedded samples were collected from the Xishanyao Formation in the Junggar Basin and prepared as composite cores (coal:sandstone = 1:2). The initial gas-bearing coal seam was 9.8 cm thick. 3 / g, sandstone 6.5cm 3 / g; Lithological differences were identified: T2 spectra showed that adsorbed gas accounted for 72% in coal seams and free gas accounted for 60% in sandstone. CO2 injection optimization: For coal seams, an injection scheme of 8MPa, 50℃, and 12h is adopted; for sandstone, an injection scheme of 4MPa, 80℃, and 12h is adopted. Results verified that the residual gas utilization rate in coal seams was 75%, and in sandstone it was 68%; the total CO2 sequestration efficiency reached 0.38t CO2 / t CH4, which is 22% higher than the unified parameter scheme.

[0028] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology, characterized in that, Specifically comprising the following steps: (1) Simulating the gas-bearing conditions of different lithologic reservoirs under abandoned conditions, determining the initial gas content of each lithologic reservoir by low-field nuclear magnetic resonance T2 spectrum test combined with centrifugation method; (2) Based on low-field nuclear magnetic resonance technology, using a low-field nuclear magnetic resonance analyzer, performing CPMG sequence scanning on each lithologic reservoir to identify the occurrence state of multi-state CH4 in each lithologic reservoir; (3) Injecting CO2 into each lithologic reservoir, recording the CO2 injection amount in each lithologic reservoir in real time through a flow sensor, synchronously collecting T2 spectrum, and monitoring the change of signal intensity of the short T2 peak representing CO2; (4) Based on the dynamic quantification of multi-phase fluid, quantitatively characterizing the phase state change of multi-phase fluid with CO2 injection time after CO2 enters each lithologic reservoir; (5) Constructing a response surface model to determine the optimal injection conditions of CO2.

2. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (1), the lithologic reservoirs include coal, shale and tight sandstone.

3. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (1), the centrifugal speed of the centrifugal method is 10000 rpm, and the centrifugal time is 2h.

4. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (2), the magnetic field strength of the low-field nuclear magnetic resonance analyzer is 0.3T, and the resonance frequency is 12MHz.

5. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (2), the multi-state CH4 includes adsorbed CH4 and free CH4.

6. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (3), the injection scheme of CO2 includes pressure gradient experiment and temperature gradient experiment.

7. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 6, characterized in that, The operation of the pressure gradient experiment is specifically: fixing the temperature at 50℃, and sequentially injecting CO2 at a pressure of 2MPa, 4MPa, 6MPa, 8MPa, and maintaining each pressure point for 2h.

8. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 6, characterized in that, The operation of the temperature gradient experiment is specifically: fixing the injection pressure at 6MPa, adjusting the temperature to 40℃, 60℃, 80℃, and maintaining each temperature point for 2h.

9. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (4), the multi-phase fluid includes adsorbed CO2, adsorbed CH4, free CO2 and free CH4.

10. The coalbed gas carbon dioxide enhanced recovery simulation method based on low-field nuclear magnetic resonance technology according to claim 1, characterized in that, In step (5), the factors of the response surface model include injection pressure, temperature and time.

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